Rix,
I found early on as a Zultys reseller, their phones default to 10ms packetization with whatever codec in the earlier releases of firmware. The newer releases I believe default to 20ms packetization. I never thought that would be an issue except for higher packet header overhead. I had the same issue where if one side (20ms SIP UA) initiated the call (to a Zip) it would work but if switched (Zip to 20ms SIP UA) it would not. You can set this via the ZIP phone's web gui of the TFTPconfig file.
Most other vendors phones do not have a good dsp/codec that can handle the higher io rates at 10ms. Can you believe that the Zip 4x4 and 4x5 had a powerpc cpu (full, not ip core) in each unit! Talk about horsepower... no wonder they went bankrupt. At the engineering level the Zip 4x5 were way ahead of their time. VxWorks RTOS, IPsec VPN, Dynamic Call Encryption, POTS ATA, Bluetooth, etc etc... I loved the PCB engineering and sheet metal design of the MX1200, MX250 chassis. However the industrial designer of the ZIP series responsible for the shell and layout should have been shot. The phones all felt cheap (nice way of putting it). Did not hold up well under high volume use. The phones were a hard sell when viewed after a Aastra, Snom or Cisco IP Telephone. I still use the 4x5 internally and still love them even with their flaws. Getting harder to keep the ZIPs arround when phones like the Snom 820 & 870 are becoming available. 8-)
-Isaac